Is selective absorption of ultrasoft x-rays biologically important in mammalian cells?

Goodhead, D.T.; Thacker, J.; Cox, R.

Physics in Medicine and Biology 26(6): 1115-1127


ISSN/ISBN: 0031-9155
PMID: 7323149
DOI: 10.1088/0031-9155/26/6/010
Accession: 043472767

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This paper tests whether photon absorption processes in particular atomic element(s) may be responsible for the observed high relative biological effectiveness (RBE) of ultrasoft X-rays. The effectiveness of titanium K characteristic X-rays (4.55 keV) is compared with previous observations for aluminium (1.5 keV) and carbon (0.28 keV) K ultrasoft X-rays. For a given absorbed dose, five times more Ti K than Al K photons are absorbed in phosphorus; since Al K X-rays are observed to be more effective in killing human and hamster cells it is concluded that absorption in phosphorus does not play a dominant lethal role. This is supported by the observation that the absolute number of Al K photons absorbed in phosphorus of DNA of human fibroblasts is less than 1 per lethal event. For no element is the relative number of absorbed photons of the three X-ray energies even approximately proportional to their observed RBEs. The effectiveness of ultrasoft X-rays is apparently not due to selective absorption but rather to the secondary electrons; consequently the mechanism of action should be common to the large numbers of low energy secondary electrons produced by most other ionising radiations, including gamma-rays.